1. Field of Application
The present invention relates to a voltage control apparatus for installation on a motor vehicle such as an automobile, truck, etc.
2. Description of Related Art
The electric generator of a motor vehicle is generally a combination of an AC generator apparatus that is driven from the vehicle engine, and a rectifier apparatus, with the electric generator thereby producing a DC voltage which is supplied to charge the vehicle battery and to drive various electrical loads of the vehicle. The higher the level of output current that must be supplied to charge the battery and to drive the electrical loads, the greater will be the level of torque that must be applied from the engine to the rotor of the electric generator (i.e., of the AC generator apparatus). In recent years, there has been a trend towards increasing the amount of electrical equipment installed on a motor vehicle, so that that total load that must be supplied by the electric generator is increasing accordingly.
However if the level of torque that must be applied to the electric generator from the engine becomes excessive, then this can result in instability of engine starting. For that reason, it has been proposed to lower the value of output voltage of the electric generator during engine starting. In that way, the level of output power supplied by the electric generator is reduced, so that the torque load on the engine is correspondingly lowered during the engine starting.
A method of performing this, using the voltage control apparatus of the vehicle (generally referred to as the voltage regulator of the generator) to reduce the output voltage of the electric generator during a predetermined interval while engine starting is being performed, is described in Japanese patent publication No. 3-173324 (pages 2-4, FIGS. 1,2), referred to in the following as reference document 1. Similarly, with a method described in Japanese patent publication No. 63-92232 (pages 2-4, FIGS. 1-3), referred to in the following as reference document 2, an ECU (electronic control unit) temporarily lowers the voltage to which the electric generator output is regulated, while such lowering is judged to be required during engine starting. In the following, such temporary lowering of the electric generator output voltage during some specific operating condition of the vehicle engine will be referred to as “restraining” the electric generator output voltage.
With the method of reference document 1 above, due to the fact that restraint of the electric generator output voltage is performed during a fixed interval that occurs during engine starting, with the fixed time interval being determined by a timer, etc., the same degree and duration of restraint of the generator output voltage will be applied irrespective of the level of electrical load that is being supplied from the electric generator. Hence, even if that electrical load is small, restraint of the generator output voltage will be applied when it is not actually necessary. As a result, if the duration of each fixed interval of restraint of the generator output voltage is long, thereby reducing the amount of charging current that is supplied to the vehicle battery, then the level of charge in the battery may become excessively low.
With the method of reference document 2 above, which changes the regulated output voltage value of the electric generator to a different value during engine starting, by a control signal supplied to the electric generator control circuit from an external apparatus (i.e., engine ECU), it can be ensured that restraint of the generator output voltage will be performed during engine starting only when it is actually necessary. However during engine starting in cold weather, the terminal voltage of the battery (during normal operation of the electric generator) may be greatly reduced. In such a condition, the necessary degree of restraint of the generator output voltage (to achieve a required improvement in engine starting performance) cannot be achieved by applying only a small reduction in the regulated value of the generator output voltage.
That problem could be alleviated, to achieve improved engine starting performance during cold weather operation, by applying a substantial lowering of the regulated value of the generator output voltage, to effect restraint of the generator output voltage during engine starting. However in that case there is a danger that the regulated value of the generator output voltage might become fixed at the very low value, as a result of some abnormality occurring in the connecting leads or connection terminals that transfer the control signal from the ECU. In such a case, almost no generator power would be produced by the electric generator, so that serious problems in operating the vehicle could arise, with the battery becoming completely discharged.
Hence with such a method, it is not practicable to set the regulated value of the generator output voltage at a very low value.